1. Field of the Invention
This invention relates to heat exchangers used in refineries and petrochemical plants. In particular, this invention relates to mitigation of fouling in heat exchangers.
2. Discussion of Related Art
Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbons-based deposits on heat exchanger surfaces. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers—thus increasing temperature in the system. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient pressure and flow in the heat exchanger.
Heat exchanger in-tube fouling costs petroleum refineries hundreds of millions of dollars each year due to lost efficiencies, throughput, and additional energy consumption. With the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Petroleum refineries and petrochemical plants also suffer high operating costs due to cleaning required as a result of fouling that occurs during thermal processing of whole crude oils, blends and fractions in heat transfer equipment. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils and blends in pre-heat train exchangers.
Fouling in heat exchangers associated with petroleum type streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of insoluble materials, and deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall.
One of the more common root causes of rapid fouling, in particular, is the formation of coke that occurs when crude oil asphaltenes are overexposed to heater tube surface temperatures. The liquids on the other side of the exchanger are much hotter than the whole crude oils and result in relatively high surface or skin temperatures. The asphaltenes can precipitate from the oil and adhere to these hot surfaces. Prolonged exposure to such surface temperatures, especially in the late-train exchanger, allows for the thermal degradation of the asphaltenes to coke. The coke then acts as an insulator and is responsible for heat transfer efficiency losses in the heat exchanger by preventing the surface from heating the oil passing through the unit. To return the refinery to more profitable levels, the fouled heat exchangers need to be cleaned, which typically requires removal from service, as discussed below.
Heat exchanger fouling forces refineries to frequently employ costly shutdowns for the cleaning process. Currently, most refineries practice off-line cleaning of heat exchanger tube bundles by bringing the heat exchanger out of service to perform chemical or mechanical cleaning. The cleaning can be based on scheduled time or usage or on actual monitored fouling conditions. Such conditions can be determined by evaluating the loss of heat exchange efficiency. However, off-line cleaning interrupts service. This can be particularly burdensome for small refineries because there will be periods of non-production.
Mitigating or possibly eliminating fouling of heat exchangers can result in huge cost savings in energy reduction alone. Reduction in fouling leads to energy savings, higher capacity, reduction in maintenance, lower cleaning expenses, and an improvement in overall availability of the equipment.
Attempts have been made to use vibrational forces to reduce fouling. U.S. Pat. No. 3,183,967 to Mettenleiter discloses a heat exchanger, having a plurality of heating tubes, which is resiliently or flexibly mounted and vibrated to repel solids accumulating on the heat exchanger surfaces to prevent the solids from settling and forming a scale. This assembly requires a specialized resilient mounting assembly however and could not be easily adapted to an existing heat exchanger. U.S. Pat. No. 5,873,408 to Bellet et al. also uses vibration by directly linking a mechanical vibrator to a duct in a heat exchanger. Again, this system requires a specialized mounting assembly for the individual ducts in a heat exchanger that would not be suitable for an existing system.
Thus, there is a need to develop methods for reducing in-tube fouling, particularly for use with existing equipment. There is a need to mitigate or eliminate fouling while the heat exchanger equipment is on-line. There is also a particular need to address fouling in pre-heat train exchangers in a refinery.